The main approaches of the simulation method for an estimation of the pressure distribution inside the vacuum chamber in the different configurations of the vacuum chamber as well as different geometry of the ion beam transport lines are considered. The results of the simulation program codes tested on the base of the data obtained at the FLNR heavy ion cyclotrons are presented. The developed simulation method practically allows optimizing any configuration of the vacuum system from the point of view of the pumping speed and pump's location. In conjunction with another program code, developed by the authors for an evaluation of the ion charge changing cross-section, this simulation method allows to forecast the ion beam loss due to the charge exchange with the residual gas molecules.