You need a strong detergent to clean the walls. The nicotine is what has yellowed the paneling. Worse case is TSP or tri-sodium Phosphate but this product is normally used to prep for painting as a cleaner. It will have a tendency to dull the original finish.
Also white vinegar and baking soda will work. Spray the vinegar on the wall with hand sprayer, Then when damp, take a mop or good rag that has been wet with the vinegar and dip it in baking soda and scrub. You will need to rinse several times as this beaks down the film on the walls. Once done rinse well with fresh water and dry so not to damage the paneling with to much water.
Wood soap or I use Awesome. Family dollar, $3.00 for half gallon undiluted. I use it all the time and best thing I have found. Safe on wood and gets all that sticky residue off. I then rinse it with clean water after words. If there is still an oder (that sometimes gets trapped in the wood) use a vinegar water solution to rinse. I use about 3/4 C. awesome to 2 quarts water.
I agree with @Shari K. I've used Murphy's Oil Soap. It's taken off wood smoke soot and nicotine off paneling for me before.
And after you get it clean....even before....stop smoking in your home.
I read years ago that vodka takes the smell out. I used it on a book case and it worked for me. Wipe item down with vodka dampened rag. Redampen as needed. Good luck
when you do use cleaners to get smoke stains off the walls...use gloves! nicotine can be a potent poison. take care to dispose of you cleaning solution in a safe manor.
use the murphys, then polish with liquid gold. it kept the paneling in my mother's home gleaming. this was one of my jobs at home, spring and fall.