Does anyone know if you can get a credit at the NEX for the unused propane tanks??

Yes, you can get 18.00 a tank.

Actually I took them to the NEX back where you pick up larger items/furniture and they give you a voucher to go to the Customer Service desk and you get $20 each gift card.

If you saved the original receipt, then you can get back the full cost of the tank, which is $34ish I believe.