Chances are, if you have a combo wd in a closet, it is because you live in a condo, and the closet is isolated from any outside walls.
Your primary goal is to get the dryer vented to the outside, to get all the moisture and lint out of your house. You cannot under any circumstances dump the vent into a crawlspace or attic, unless you can continue the vent all the way to the outside.
Unfortunately, most combo units are not as brutish as ordinary dryers, so you are going to have to be careful about how long the run goes, and not too many bends, and not too much "lift" (venting the dryer 8' above the dryer itself is not desirable) It would be helpful to get the actual venting info from your dryer manufacturer. I don't know who makes yours, but GE's customer service number is 1800-626-2000.
If it turns out you absolutely cannot vent this to the outside, I think the plastic box you have there is a crude replacement concept. The idea is to use it as a trap, where the moisture and lint go and then you clean the box out on a regular basis. I don't remember if you are supposed to have water in the box or not, but I think you do.
In fact, GE might have some ideas for you as well. Give them a shout.