6 (6- to 8-ounce) center-cut skin-on salmon fillets
2 cups wood chips
Combine sugar and salt in bowl. Set salmon on wire rack set in rimmed baking sheet and sprinkle flesh side evenly with sugar mixture. Refrigerate, uncovered, for 1 hour. With paper towels, brush any excess salt and sugar from salmon and blot dry. Return fish on wire rack to refrigerator, uncovered, while preparing grill.
Combine soaked and unsoaked chips. Use large piece of heavy-duty aluminum foil to wrap chips into foil packet and cut several vent holes in top. Place wood chip packet directly on primary burner. Turn primary burner to high (leave other burners off), cover, and heat grill until hot and wood chips begin to smoke, 15 to 25 minutes. Turn primary burner to medium. (Adjust primary burner as needed to maintain grill temperature of 275 to 300 degrees.)
Clean and oil cooking grate. Fold piece of heavy-duty foil into 18 by 6-inch rectangle. Place foil rectangle over cool side of grill and place salmon pieces on foil, spaced at least ½ inch apart. Cover grill (positioning lid vent over fish if using charcoal) and cook until center of thickest part of fillet registers 125 degrees and is still translucent when cut into with paring knife, 30 to 40 minutes. Transfer to platter and serve, or allow to cool to room temperature.