Pour 1 cup Chili-Lime Marinade into a large zip-top plastic freezer bag; add prawns. Seal bag, turning to coat prawns. Chill 30 minutes.
Pour remaining 1 cup marinade into a large bowl. Prepare noodles according to package directions. Add noodles to marinade in bowl, tossing to coat. Cut hearts of palm in half crosswise, then lengthwise into thin strips; add to noodles. Add onion to noodle mixture, and toss gently.
Preheat grill. Remove prawns from marinade, discarding marinade. Grill prawns, covered with grill lid, over medium heat (350° to 400°) for 2 minutes on each side. Place noodle mixture on each of 6 individual plates, and top each with prawns and tomatoes. Garnish, if desired.