Natural gas is a promising alternative gaseous fuel due to its availability, economic, and environmental benefits. A solution to increase its use in the heavy-duty transportation sector is to convert existing heavy-duty compression ignition engines to spark-ignition operation by replacing the fuel injector with a spark plug and injecting the natural gas inside the intake manifold. The use of numerical simulations to design and optimize the natural gas combustion in such retrofitted engines can benefit both engine efficiency and emission. However, experimental data of natural gas combustion inside a bowl-in-piston chamber is limited. Consequently, the goal of this study was to provide high-quality experimental data from such a converted engine fueled with methane and operated at steady-state conditions, exploring variations in spark timing, engine speed and equivalence ratio. The results showed that a higher engine speed reduced the motoring pressure, advanced maximum brake torque timing, and reduced the power output per cycle. Moreover, advanced spark timing increased and advanced the cylinder pressure, and increased both hydrocarbon and nitrogen oxides emissions. Leaner operation retarded the flame development process and decreased the cylinder pressure because of the lower energy per cycle. Furthermore, advanced spark timing can produce a dual peak in the heat release, a unique characteristic of such converted engines. Finally, the variation of the peak cylinder pressure and CA50 were relatively small, indicating that the data in this study can be used for numerical simulations. In addition, there was no knocking phenomena during experiments.