Heat the oven to 400°F and grease a 3-quart baking dish.
Steam the cauliflower for 5 minutes, until barely tender, then spread it on a clean towel and pat it thoroughly dry.
Beat the cream cheese, hot sauce, garlic powder, salt and pepper in a large bowl until smooth.
Fold in the cauliflower, chicken, celery and 1 cup Monterey Jack and spread the mixture in the baking dish.
Top with the remaining cheese and bake for 20 to 25 minutes, until bubbling and 165°F at the center.
Rest for 5 minutes before serving so the sauce clings instead of running.