A system and method for determining the quality of natural gas is provided. The system includes a fuel line for communicating natural gas to an engine. An infrared light source is disposed along the fuel line and is configured to emit a beam of infrared light into the fuel line having a wavelength of 6 to 10 micrometers. An infrared light detector detects a transmission value of the natural gas as the beam of infrared light passes through the fuel line. A natural gas quality module receives the transmission value from the infrared light detector and determines a quality value of the natural gas based on an amount of infrared light absorbed by methane in the natural gas. An engine control module, including a feed-forward control loop, receives the quality value from the natural gas quality module and alters an operating parameter of the engine in response thereto.