Is it possible to subject plant leaves to bacterial digestion to harness methane production therefrom. Is so what are the limitations.
Yes, plant leaves, like pretty much all other organic matter, can be digested by the right bacteria to produce methane, and a nutrient-rich byproduct which can be reused in agriculture as fertiliser.
The standard method is to used anaerobic digestion in a closed unit. When done well, this cuts out the risk of fugitive methane emissions, and that's very important, because (as blacksmith37 notes), it's a very potent greenhouse gas.