Prepare grill to medium heat.
Cut plantains in half lengthwise. Cut plantain halves in half crosswise. Combine butter, sugar, and pepper; brush evenly over cut sides of plantain sections.
Place plantain sections, cut sides up, on grill rack coated with cooking spray; grill 7 minutes or until flesh is soft and skins begin to pull away from the flesh. Turn plantain sections over; grill 3 minutes. Serve warm with lime wedges.