Cook rice mix according to the package directions using the seasoning packet. In a large mixing bowl, combine cooked rice, chicken, soup, mayo, celery, onion, sweet pepper, and water chestnuts. Mix well. Spread into an ungreased 3 qt baking dish. In a small bowl, combine bread crumbs and butter. Toss to coat. Sprinkle over casserole. Bake @ 325 for 45-50 minutes or until crumbs are toasted and casserole is heated through.