The development of multi-petawatt laser systems opens up the way to observe electron-positron pair production in various circumstances. In this study, we consider a gas of neutral xenon, the electrons of which get ionized and accelerated. These seed particles emit high-energy photons which subsequently decay producing electron-positron pairs via the Breit-Wheeler mechanism. Here we investigate the conditions for efficient generation of pairs in the presence of a circularly polarized ultra-intense laser pulse (or several pulses). A special focus is placed on the analysis of the magnetic field effects and enhancement of the pair-production process due to the circular polarization of the laser field.