Preheat oven to 350 F. Cook pasta according to package directions, drain. Melt butter in a large saucepan over medium heat. Add in flour and cook for a minute to brown. Add in milk, chicken broth, salt, pepper and garlic powder.
Continue to cook over medium heat, while whisking constantly until thickened. Mix sauce with pasta along with chicken, peas, corn and green beans. Spread mixture out into a large greased baking dish. Sprinkle with shredded cheddar cheese. Bake for 25-30 minutes.