Use the point of a spudger to disconnect each of the four antenna connectors from the AirPort/Bluetooth card.
Note the original positions for each connector when reconnecting the cables. From left to right on the card, the antennas are connected in the following order:
Pull the hard drive SATA cable and connector through the right hard drive bracket. Move the cable to the right side of the iMac, out of the way of the exhaust port.
Don't attempt to completely remove the cable—it can't be removed just yet, but will get in the way of removing the logic board.
When reassembling your iMac, be very careful to align the exterior I/O ports correctly. The logic board can sit crooked even when secured with all its screws.
You can use a USB flashdrive or ethernet cable to ensure the logic board is seated correctly while you screw it in.