  The radiative transfer problem is considered in dense gas-dust clouds.
Physical parameters are adopted in simulations corresponding to gas-dust clouds
in the circumstellar envelopes of late-type stars. An one-dimensional
plane-parallel slab geometry of the cloud is considered. The radiative transfer
problem is solved using the accelerated lambda iteration technique. The
rotational levels of the five lowest vibrational levels of H2O-molecule are
considered in the simulations. The level populations of H2O-molecule are
computed as a function of distance in the cloud. The conditions of the level
population inversion occurrence are investigated. In particular, the factors
controlling the population inversion in the 22.2 GHz maser line are considered.
A radiative excitation of the molecules by external radiation field of a star
is considered together with the collisional pumping of the maser.
