Here is another option. It takes a while, but you don't have to worry about melting plastic.

Sherri's MB CL600 had VERY gummy buttons on the steering wheel. Same type issue with the soft plastic outgassing or something. I read a post on one of the MB websites about cleaning with car wax. Most car waxes have a cleaning agent but will not hurt plastic or rubber IF you clean off all the wax when done. I worked on those buttons for about 30 minutes, and slowly but surely they were clean, smooth and not a hint of stickiness. That was 6 mos ago, and they remained clean and smooth without any stickyness until I sold it a couple weeks ago.

I used Zymol, just because that's what I had laying around, but any good car was would work. I tried a couple spots on my comfort grip wheel, and it came clean very quickly. Of course, because of the grain, you have to use a good terry cloth (maybe even a tooth brush) to make certain you remove all residue. I have not had time to do the whole steering wheel (car is "trapped" on the lift right now) but will when I get a chance.