The conversion of heavy-duty diesel engines to spark-ignition natural gas operation has the potential to decrease the use of conventional petroleum-based fuels and reduce greenhouse gas emissions. A better understanding of the fundamentals such as early natural gas flame development in re-entrant bowl combustion chambers can accelerate this conversion. This paper details an optical investigation of flame luminosity inside a conventional heavy-duty diesel engine converted to spark-ignition natural gas operation by replacing the diesel fuel injector with a spark plug and adding a port-fuel gas injector in the intake manifold. Knock-free lean-bum experiments were performed at medium engine load using methane as fuel. Combustion images confirmed that kernel inception played an important role in the subsequent flame propagation. In addition, flame luminosity images of individual engine cycles showed strong flame wrinkling and counterclockwise rotation due to increased turbulence inside the re-entrant bowl. However, the flame front for the mean cycle was relatively circular. Flame luminosity also suggested a thick flame and a high turbulent flame speed for early inside-the-bowl flame propagation, at the operating conditions investigated. Higher surface-to-volume ratio in the squish region increased the heat transfer to the surroundings and reduced flame propagation, which increased the late combustion period. The separation of the combustion process into two distinct zones (i.e., inside and outside the piston bowl) created a secondary peak or "bump" in the heat release of individual cycles. The data suggests that the combustion strategy should optimize the mass of fuel that bums inside the squish region. In addition, the moderate rate of pressure-rise and lack of knocking showed promise for heavy-duty diesel engines converted to spark ignition natural gas operation.