  Networks of reactions on dust grain surfaces play a crucial role in the
chemistry of interstellar clouds, leading to the formation of molecular
hydrogen in diffuse clouds as well as various organic molecules in dense
molecular clouds. Due to the sub-micron size of the grains and the low flux,
the population of reactive species per grain may be very small and strongly
fluctuating. Under these conditions rate equations fail and the simulation of
surface-reaction networks requires stochastic methods such as the master
equation. However, the master equation becomes infeasible for complex networks
because the number of equations proliferates exponentially. Here we introduce a
method based on moment equations for the simulation of reaction networks on
small grains. The number of equations is reduced to just one equation per
reactive specie and one equation per reaction. Nevertheless, the method
provides accurate results, which are in excellent agreement with the master
equation. The method is demonstrated for the methanol network which has been
recently shown to be of crucial importance.
