Place whole chicken in large pot or Dutch oven and cover with water. Bring to a boil, reduce heat, and simmer 40 to 60 minutes, until tender. Remove chicken from pot, reserving broth. Skin and bone chicken; set meat aside.
Preheat oven to 350 degrees F (175 degrees C). (For slow cooker instructions, see Cook's Note.)
Bring chicken broth to a boil. Stir in pasta and cook 8 to 10 minutes, until al dente. Remove pasta from pot and set aside.
In a large skillet over medium heat, melt butter. Cook celery, garlic, onion and bell pepper in butter until tender. Toss celery mixture with pasta, diced tomatoes and green chiles, processed cheese, sliced mushrooms and chicken meat. Place in 9x13 inch baking dish. Pour reserved chicken broth over to moisten.
Bake in preheated oven 20 minutes, until hot and bubbly.
