could this cause me problems as I seek publication?

That's nearly half the project being posted online. I'm going with it's a bad idea if you expect to shop it around for publication; why would somebody take it on if you've already given half of it away? If it was 250 words, yeah, you'd probably be fine -- it's a small enough bite to be considered a decent sample -- but not with 25,000.

It's not a great idea. Can you remove it online?

Sounds like a bad idea to me, too...

Also, I would ask whether you've clarified your own goals for your writing.

If, as you state, you're impatient for feedback and exposure and want to share it NOW, NOW, NOW.... traditional publishing may not be your thing. Consider alternate outlets - or if you'll be just as happy sharing your work online for free. (I'd suggest self-publishing, but to do that right takes a lot of revising and editing and hustling which can cause a delay in release, with the trade-off of a superior product. On the plus side, you and you alone would control release schedules and marketing and everything else, and you would eliminate the obstacle of finding a publisher and/or agent to rep your work.)

If you mean to go the traditional route, you're going to have to learn some patience, with the Process and with yourself.

There is nothing inherently wrong with writing for immediate feedback. It's your writing, your art, and you are the only one who can decide what it is for, what would bring you the greatest satisfaction. Just do not expect to have your publication cake and eat immediate feedback too, at least for the same confection.

As a compromise... have you considered writing some intentional freebies, maybe flashfic (1000 words or fewer), for sharing ASAP while honing your publication-bound work? That might scratch the impatience itch while still allowing you to move toward publication.

JMHO...

Sorry, but I'm afraid I must agree that it might be a plan to rethink. I admire your enthusiasm, but don't overlook patience. All I've heard is that once something is "published" (typically self), the traditional book industry won't be terribly enthused unless you can show really huge sales figures. At that point, you wouldn't need a traditional publisher! For what it's worth, same applies to series books -- once one has an ISBN someplace, it's a very difficult path in a very competitive marketplace to switch into traditional.

My experience has been that getting into traditional publishing with anyone halfway decent is an effort requiring great patience and perseverance matching your work with the tastes of an agent and then an acquisitions editor and then the editorial board of a house always anxious about potential money-losers. I'm currently 0-for-3 and working on #4 and it seems like it's been forever since it all began. Finish your project, polish it the best you can, begin the endless task of queries and start writing something fresh in the meantime.

Good luck ... and enjoy the craft.

This seems like a question for an agent who deals with literary fiction in particular. While Narrative's content is free, you do have to "sign up" to view it, which to my mind makes it less accessible than something on Wattpad or whatever. The people who read it are likely already to be Narrative readers. And it is a top journal, so publication there means something.

As you say, short story collections usually consist of previously published material. But I'm not sure how that works with a sizable chunk of a novel. Have previous novels that were excerpted in Narrative gone on to be trade published?

There is plenty of precendent for novels that were based on shorter pieces that got published. If you can get Narrative to take your novella that i think it would enhance the chances of the novel.

Wasn't there an agent who was interested in this collection? If that's still the case, I would ask him/her before moving forward with submitting.

I don't think it would be a problem, but I would also probably be pitching it as a novel-in-stories rather than a traditional novel. Then again, I'm not an agent, and I second Fuchsia Groan's suggestion for seeing what you can find re: lit fiction agents and how they approach this sort of project.