Mix the brown sugar, salt, black pepper, onion powder, garlic powder, celery seeds, and white pepper in a small bowl until thoroughly combined, and set aside.
Remove any small bones from the salmon that might have been missed when filleting. Put it in a shallow bowl and cover it with the dry brine. Press down gently, making sure the filets are completely covered.
Let it brine in the fridge for 4-24 hours, depending on the thickness of the filet and the desired saltiness.
Thoroughly rinse the brine off the salmon under cold running water to remove excess salt. Pat dry with paper towels.
Place it skin-side down on a rack on top of a baking sheet, and place it in the refrigerator, and let it dry for at least 4 hours (preferably overnight) until the surface is lightly tacky. That helps the smoke flavor adhere to the fish.
Preheat the pellet grill to 180-225°F (80-110°C) with the lid closed for 15 minutes.
Remove the salmon from the fridge, sprinkle with optional seasonings, place the filets, skin-side down, directly on the grates, and insert a meat thermometer into the center.
If the smoker is at 225°F (110°C), it may take 45 minutes to 1½ hours. If you're smoking it at 180°F (80°C), it may take up to 4 hours. You'll want an internal temperature of 140-145°F (60-63°C) for tender, flaky fish.
Remove it from the smoker and serve it warm with lemon wedges or chilled.