Boil chicken breast, celery, carrots, and onions for 1 1/2 hours.
Spoon chicken and vegetables into large mixing bowl. Pull hot chicken apart with knife and fork. Caution: chicken will be boiling hot. Cook macaroni only in hot chicken broth for 8 minutes.
While macaroni is cooking; add cream of chicken soup, corn, and green beans to cooked chicken and vegetables. Stir well, add salt and pepper to taste.
Drain macaroni, saving 1 cup of the liquid. Add cooked macaroni to the chicken and vegetable mixture. Mix well. Add packet of powdered cheese into the mixture. Stir well until dissolved. Add reserved broth to make mixture very moist. MAKE SURE CASSEROLE IS VERY MOIST BEFORE BAKING OR IT WILL BE A DRY CASSEROLE. Pour into large casserole dish, sprinkle with bread crumbs.
Bake in a preheated 300 degree oven for 45 minutes or until toasty brown and bubbly. Take casserole out of oven and let sit for 15 minutes before serving.