The orange peel is going to decompose and produce CO2 either way. Methane is produced when there is not enough oxygen available while decomposing, which certainly seems a possibility if it's dumped in big piles.
Remember the orange trees took the CO2 out of the atmosphere to make the peels. Some of it, probably most of it, is going back into the atmosphere but some of it is going to become soil carbon which could be retained for decades
