Large-eddy simulation of the reacting flow field in a combustion-based mitigation system to reduce the emissions of methane contained in ventilation air methane is presented. The application is based on the preheating and combustion of ventilation air methane. Effects of preheating and methane concentration are examined in five computational cases. The results indicate that the oxidation of the ventilation air methane can take place in a co-annular jet configuration provided that the preheating temperature is as high as 500 K for mixtures containing a low methane concentration of 0.5%. It is found that the oxidation process that eventually leads to reaction and combustion is controlled by the methane concentration and the level of preheating.