How to mask port numbers behind a reverse proxy

This is probably a really dumb question. I've got multiple Rails 3 apps
behind a nginx reverse proxy that are working okay. My problem is that
most of the hyperlinks are appearing with the application port numbers
on them - they work fine, but I was hoping to serve everything up on
port 80 so everything looks like the same site.

What I was looking for was a setting like HOST='something.com/thisapp'
in the configuration so that the generated pages won't have the port
number in them. I've found just a few posts on this - but nothing has
really worked so far for me.

Am I missing something really obvious or just wasting my time with this?