[en] The ATLAS detector is designed to exploit the full potential of the LHC, identifying and providing highly accurate energy and momentum measurements of particles emerging from the LHC proton-proton collisions. High-momentum final-state muons are among the most promising signatures at the LHC, thanks to a high-resolution Muon Spectrometer with standalone triggering and momentum measurement. The Muon Spectrometer is a large and complex system of gaseous detectors, requiring a detailed simulation to exploit its full capabilities. Over the last few years these systems have been described in terms of a set of geometrical primitives known as GeoModel. This description is now used in the GEANT4-based simulation program, which is fully operational and integrated into the ATLAS common analysis framework