Use of natural gas-diesel dual-fuel (NDDF) combustion in compression ignition engines is a method of reducing the net greenhouse gas (GHG) and particulate matter (PM) emissions of these engines. Compressed natural gas (NG) is injected into the intake manifold of the engine and the air-NG mixture is ignited by a direct injection of diesel in the cylinder. One of the main challenges with NDDF combustion is the methane (primary component of NG) slip at low and medium loads, which reduces the engine efficiency and offsets the advantage of lower carbon dioxide emissions of the NG combustion. In order to address this issue, an intake manifold insert is devised with the objective to alter the intake flow profile into the engine and ultimately reduce the methane slip. This is a novel strategy for an NDDF engine since modifying the in-cylinder flow profile can intensify the mixing between diesel and air-NG mixture in order to improve the NG utilization in the cylinder.Tests are conducted without and with the insert immediately upstream of the intake valves to compare diesel and NDDF combustion at a medium engine load and low engine speed. A single-cylinder, four-stroke, heavy duty research engine is used for the tests. In the absence of direct flow measurement, three-dimensional numerical simulations, performed with a computational fluid dynamics software suite, are used to estimate the changes in the in-cylinder flow profile with the use of the insert.NDDF tests indicate that the use of the intake insert can decrease the methane slip (by ~28%) and carbon monoxide emissions (by ~25%) at the expense of increase in the emissions of nitrogen oxides (NOx - by ~23%) and decrease in the indicated thermal efficiency (~1.7%). Use of the insert causes the GHG emissions to increase and decrease for diesel and NDDF combustion respectively. Additionally, use of the insert causes the NOx emissions to remain similar and increase for diesel and NDDF combustion respectively. By using low levels of exhaust gas recirculation, the NOx emissions of NDDF combustion can be reduced without significant impact on the GHG or PM emissions when compared to diesel combustion.