In a small bowl, combine brown sugar and cornstarch.
Stir in the water, soy sauce and reserved pineapple juice until smooth; set aside.
In a large nonstick skillet or wok, stir-fry carrot in 1 tablespoon hot oil for 1 minute. Stir in the celery, sweet peppers and onion; stir-fry for 3 minutes.
Add sugar snap peas; stir-fry about 1 minute longer or until vegetables are crisp-tender. Remove vegetables and keep warm.
In the same skillet, stir-fry chicken in remaining oil for 3-4 minutes or until no longer pink.
Stir soy sauce mixture; gradually stir into skillet.
Bring to a boil; cook and stir for 1-2 minutes or until thickened.
Stir in vegetable mixture and pineapple chunks; heat through.