Reality Factory

First I am unsure if this is in the right place, but it has something to do with modeling programs.

In Nem's terrain generator I made a landscape, and discovered that it causes that cause RFedit to slow down due to too many faces, so I export the (Landscape) geometry as a .3ds file, go into the Gmax "tempest gamepack" and remove the unneeded faces. I export it from there as a ".map" file. When I go to import into RFedit, the map won't load. Not even an error message.

Moved to Level Design & Entity Usage as this has more to do with level design than modelling.

Either Gmax does not export to a map format that is compatible with the RF editor or you caused invalid brush definitons by removing the faces, so the editor can't render them.

Well I got something that helps, I'm using nem's TG in my game too, so what you could do:
if there are triangles that are forming a straight plane, replace them with a cube brush, mark all unseen faces as Sky, make the triangle brushes details and set the light map scale to a highr value like 7 or 10 or even higher. This helps me a lot with my game