In a saucepan, bring 5 cups of water to a boil. Add rice and stir (just once). Reduce heat, cover and simmer for 20 minutes. Set aside. Melt 1/2 TBSP margarine in a medium skillet over medium heat; saute onion and celery until tender.
Preheat oven to 350 degrees F.
Combine the chicken, rice, onion, celery, mayonnaise and soup in a large bowl, mix well. Transfer the mixture into a 9"x13" baking dish. Top with crushed cornflakes and 2 TBSP melted margarine.