Recently a new high-power terahertz FEL has been put in operation at the Siberian Center for Photochemical Research in Novosibirsk. The first lasing at the wavelength near 140 micrometer was achieved in April 2003. Since then some experimental data were obtained which required theoretical explanation. In this paper we use a simple 1-D model for numerical simulations of the FEL operation. The model is based on excitation of multiple longitudinal radiation modes by charged discs. We restrict our consideration to only the fundamental transverse mode. This approximation is valid in the case of long-wave FELs. We compare the results of numeric simulations with some analytical estimates and experimental data.