In 5-quart saucepan or Dutch oven, stir together beans, zucchini, tomatoes, corn, lima beans, salsa and chili powder. Heat to boiling. Reduce heat; cover and simmer 10 minutes. Spoon into baking dish. Arrange polenta on top, overlapping slices slightly if necessary. Bake 25 minutes or until bubbly at edges. Sprinkle with cheese; bake 3 minutes longer or until melted. Let stand 10 minutes before serving.