In-situ laser-absorption measurements of CO, CO2, CH4 and H2O were demonstrated in the synthesis gas products of coal gasification (here called syngas) from an engineering-scale transport reactor. A wavelength-scanned, wavelength-modulation spectroscopy scheme was used to counter environmental challenges including severe loss in transmitted light intensity due to particulate scattering and beam steering in the synthesis gas stream. Separate lasers were used for each species (CO near 2326 nm, CO2 near 2017 nm, CH4 near 2290 nm, and H2O near 1352 nm) and a novel fiber bundle was utilized to combine all four beams on a common optical path. Line-of-sight laser absorption utilized sapphire windows in the synthesis gas product pipe downstream of the gasifier reactor by approximately 5 s flow time. Time multiplexing enabled low-noise measurement of the transmitted light with a single detector. Successful measurements of the four species mole fractions were performed throughout the 54 day measurement campaign, including a significant period of unattended operation. The mole fractions of the four target species were monitored versus time as the gasifier was transitioned from a cold startup to stabilization at the target 15 atm and 1260 K gasification condition, and after 35 days at relatively steady operating conditions, the campaign was terminated and the reactor shut down. The variations of time-resolved mole fractions of the four species were captured during the heating of the reactor with a propane/air flame, the continued combustion heating of the reactor as coal was added to the fuel, and the transition from combustion to gasification. The average values of CO, CO2 and CH4 measured by the laser absorption sensor were in good agreement with extractive gas chromatography; however, the extractive gas analysis was delayed by similar to 20 min and had a similar to 15 min time response due to the transport and conditioning (filtering and drying) of the sampled gas. The laser absorption sensor provided essentially simultaneous species mole fractions with sub-second time resolution, which enabled identification of a wide range of dynamic behaviors. For example, small changes in the reactor conditions due to batch feeding of coal were observed by composition changes in the synthesis gas and were correlated with small temperature fluctuations in the reactor. (C) 2015 Elsevier Ltd. All rights reserved.