You have to choose one or the other.  You should choose to quit smoking and understand that you are going to gain weight no matter what you do. If you are not comfortable with that, then go on a diet and lose weight so you can quit smoking and gain some of it back.  It will take a good year to accomplish both. Sorry but those are the facts.  Been there, done that, but after i got sick. Do it now if you can.  Good luck