Boil the chicken in a large saucepan with the whole bell pepper, whole onion, and stalks of celery. Season the water with salt, pepper and a chicken bouillon cube. Boil over low heat until chicken is tender. Cool. Remove the vegetables and chicken from the broth. Debone the chicken. Save the broth.
Cook the spaghetti in two quarts of the chicken broth. Drain but do not rinse.
Saute the chopped vegetables in a mixture of the margarine and 1/2 cup of the chicken broth until they are tender.
Add the tomatoes and cream of mushroom soup. Simmer about 15 minutes to let the flavor blends. Add the cheese until it melts.
Add the chicken and the spaghetti. Pour into a 9 x 13-inch pan. Bake at 350 degrees for 30 minutes.