How can I reconvert a natural gas fire from propane back to natural gas?
I googled 'Napoleon B36DFN LP conversion' and the very first hit is for page 17 of the user manual for that fireplace.  See here:   http://bit.ly/1HHpEP5

It states in relevant part:  "To convert from Propane to Natural Gas order conversion kit # W175-023"

Googling for 'W175-023 conversion kit' yields a link to the kit on Amazon...  $42.  http://amzn.to/1Dgwzie

If I were you, however, I'd certainly do a bit more detective work to make sure it will work for you.  Good luck.