Yeah...that'll happen. Smoking ups your metabolism, so when you quit, you will naturally gain weight. It may not seem like a fair trade, but quitting smoking was a good idea. As for exercise (I am no expert), do exercises that keep your metabolism up, like cardio and stuff like that, so it'll act like the cigarettes did only in a healthy way.
That's because you've taken up the habit of now over eating. things taste a lot better after you give up smoking, and in place of a cigarette you eat something. one thing is almost as bad as the other.
you'll have to start counting calories and try to do some exercising.
same thing happened to me about 35 years ago. in order to cope with that weight gain (i hate to diet) i actually gave up lunches. to this day i have not changed that pattern, and so i still weigh about 20 pounds less than i did when i first started. about everything you do in life calls for some kind of will power to get you through the tough times.