Place patties in center of grid over medium, ash-covered coals. Grill, uncovered, 8 to 10 minutes (over medium heat on preheated gas grill, 7 to 9 minutes) until meat thermometer inserted horizontally into center registers 160°F, turning occasionally. About 2 minutes before burgers are done, place buns, cut-side down, on grid. Grill until lightly toasted. During last minute of grilling, top each burger with cheese.