Natural Gas (NG) is currently a cost effective substitute for diesel fuel in the Heavy-Duty (HD) diesel transportation sector. Dual-Fuel engines substitute NG in place of diesel for decreased NOx and soot emissions, but suffer from high engine-out methane (CH4) emissions. Premixed Dual-Fuel Combustion (PDFC) is one method of decreasing methane emissions and simultaneously improving engine efficiency while maintaining low NOx and soot levels. PDFC utilizes an early diesel injection to adjust the flammability of the premixed charge, promoting more uniform burning of methane. Engine experiments were carried out using a NG and diesel HD single cylinder research engine. Key speeds and loads were explored in order to determine where PDFC is effective at reducing engine-out methane emissions over Conventional Dual-Fuel which uses a single diesel injection for ignition. PDFC has shown significant reductions in methane as well as CO emissions when compared with Conventional Dual-Fuel combustion. Medium loads enjoy the largest benefit from PDFC. At higher load, the amount of diesel injected early was limited in order to stay within the constraints for cylinder pressure. At lighter loads, the benefits of PDFC are reduced due to higher in-cylinder global lambda (λ) and lower temperature and pressure to promote auto-ignition. Overall, the PDFC mode of operation observes the potential to minimize the need for exhaust after-treatment as well as increase engine efficiency.