Cook the penne pasta in boiling water seasoned with 1 tablespoon of salt. Follow the package directions for timing. Drain well and let cool.
In a large serving bowl, combine the chicken, diced tomatoes, onion, celery, and parsley.
Add the pasta to the bowl and toss to combine the ingredients.
In a small bowl, combine the mayonnaise, lemon juice, garlic powder, and pepper and whisk to blend.
Add the dressing to the pasta mixture and toss until well blended. Taste and add salt, to taste and adjust the mayonnaise and other seasonings as desired.