Trajectories are used to investigate how anharmonicity in the intramolecular potential affects intermolecular energy transfer in thermal collisions between Ar and highly vibrationally excited CH4. For impact parameters between 0 and 5 Å anharmonicity in the methane intramolecular potential does not affect the probability of intermolecular energy transfer. Results at larger impact parameters are inconclusive since there is an ambiguity in assigning small energy-transfer events as elastic or inelastic. If small energy-transfer values are neglected, the probability of relative translational energy transfer is well represented by a biexponential function, while the probability for rotational energy transfer is similar to a Gaussian. A quasiclassical trajectory model indicates that the Ar + CH4 energy-transfer cross section is significantly larger than the commonly used Lennard-Jones value.