The yellow discoloration most likely is just the aging of the plastic and can't be removed by cleaning. Rubbing alcohol will most likely remove the sticker residue.

You could try using whatever it is you're using to get rid of sticker res, marker, etc. on the yellowing.  If you got it second hand, it could be caked on dust, nicotine residue, etc.  Don't expect it to work, just give it a whirl and think of it as a bonus if it does come clean

Have you tried your luck with a Mr. Clean magic eraser?  Thats the first thing I pick up when I get a new console in, works better than anything else I've tried.  If its from smoking, I bet it will make a difference.  If its from natural yellowing from cheap plastic, you would probably be better off taking it apart and painting it with some Krylon fusion.  Nothing seems to help the yellowing of NES and especially SNES systems.

I poured Goo Gone all over it. No effect.
I'll try that Mr. Clean Magic Eraser next.