Continue along and you'll see the neurotoxin generator. Follow Wheatley up the stairs. You'll see a bunch of turrets getting destroyed. At the end of the path, an elevator will take you up.
Up ahead, Wheatley has found a door, but it's locked and he can't hack it. Turn around to find a red button. Press it. The panels that are being carried back and forth will stop, then fall. You'll see a white wall to the right that the laser is shining on. Create a portal where the laser is shining. Go through the "Implosion Observation Annex" door that is now open.
Go to the end, where you can see the neurotoxin generator. Notice some white walls going up. Create a portal of the opposite color to make the laser come through and cut the pipes connected to the neurotoxin generator. After all pipes are cut, the neurotoxin generator will implode. Go back to the office where Wheatley is. You and Wheatley and some confetti will be pulled through the tube. You'll end up in a stairwell after a couple of loading screens.