Answer: Well, there is such a word -- the dictionary says so. But the meaning is often confused with the verb nauseated. Nauseated is when you or your character feel sick. Nauseous is when you or your character smell so bad that you make other people feel sick. Same meaning as the more common variant, noxious.
Therefore, despite the fact that I've seen perfectly good authors, Dick Francis and Lisa Scottoline, among others, use nauseous to mean nauseated, and that the coloquialism is fast becoming standard usage in the US, in correct English a character cannot "feel nauseous." If he IS nauseous, it means he makes other people feel sick. If HE feels sick, he is nauseated.
Hope that answers the question.