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Controlling start of ignition in Homogenous Charge Compression Ignition (HCCI) engines remains a major challenge. Here we have investigated changes in intake charge composition and its effects on ignition delay for natural gas based HCCI engine operation. In particular, we have investigated the effects of small amounts of nitrogen dioxide (NO2) on operating characteristics. Previous research had shown that NOx presence might attenuate natural gas ignition. The hypothesized catalytic effect of NOx on methane ignition at HCCI conditions was experimentally confirmed in a custom built engine. The problem was further studied in both zero and multidimensional numerical engine simulations with detailed chemistry. The simulations were used to complete a reaction rate sensitivity analysis to elucidate the controlling chemistry, and further confirm that a significant shift in ignition phasing is produced with the addition of just several ppm by volume of NO2 or NOx (NO + NO2).