We report the first tunable diode laser (TDL) absorption measurements for gas temperature and species concentrations in a pilot-scale, high-pressure, entrained-flow, oxygen-blown, slagging coal gasifier with pressures up to 18atm and temperatures to 1800K. Three locations in the gasifier flow were investigated: (1) the reactor core, (2) the exit of the reactor where a water spray quenches the reaction products (pre-quench), and (3) in the gasifier product stream before particulate clean-up (post-quench). Gas temperature was measured by three pairs of H₂O transitions with distinct temperature dependence at all three locations using five distributed feedback (DFB) lasers near 1.4μm. CO, CO₂ and H₂O concentrations were measured at the post-quench location using DFB lasers near 2.3μm, 2.0μm and 1.4μm, respectively. Fixed-wavelength, 1f-normalized, wavelength-modulation-spectroscopy with second-harmonic detection (WMS-2f) was used for all the measurements. Usable signal-to-noise ratio (SNR)>10 was achieved with one second averaging even at gasifier conditions with large particulate loading where the non-absorption transmission loss was >99.99% from light scattering and the thermal emission was intense. The work demonstrates the capability of TDL sensors to monitor transient changes in gas temperature providing a potentially valuable control sensor for gasifier performance. In addition, measurements of CO, CO₂ and H₂O in the gasifier product stream showed good promise for monitoring the syngas heating value. The potential advantages of using higher WMS harmonics under high-pressure conditions were also investigated.