Two diagnostics were developed that are particularly suitable for use with natural gas-fuelled reciprocating engines that are used for power generation applications.The first diagnostic relates flame chemiluminescence to thermodynamic metrics relevant to engine combustion - Heat Release Rate (HRR) and in-cylinder bulk gas temperature. Studies were conducted in a single-cylinder natural gas-fired reciprocating engine that could simulate turbocharged conditions with Exhaust Gas Recirculation. Crank-angle-resolved spectra (266 to 795 nm) of flame luminosity were measured for various operational conditions by varying the ignition timing for MBT conditions and by holding the speed at 1800 rpm and Brake Mean Effective Pressure (BMEP) at 12 bar. The effect of dilution on CO₂* chemiluminescence intensities was studied, by varying the global equivalence ratio (0.6 - 1.0) and by varying the Exhaust Gas Recirculation rate. Though the measured CO₂* chemiluminescence intensities did not correlate with in-cylinder bulk gas temperature and Heat Release Rate (HRR) on a crank-angle-resolved basis, the peak values correlated extremely well with the peak cycle heat release rates and to a lesser extent with the peak cycle temperatures. Such observations point towards the potential use of flame chemiluminescence to monitor peak bulk gas temperatures, as well as, peak heat release rates in natural gas-fired reciprocating engines.A second diagnostic based on Laser-Induced Breakdown Spectroscopy (LIBS) was evaluated to measure in-cylinder equivalence ratio. Following the laser-induced spark ignition event in a single-cylinder natural gas-fuelled engine that was equipped with an Exhaust Gas Recirculation (EGR) system, the broadband emission from the spark kernel was collected and spectrally resolved. The peaks corresponding to Hα, N and O atoms were measured for a range of conditions with global equivalence ratios ranging between 0.6 and 1.0, and for Exhaust Gas Recirculation fractions up to 29%. The (Hα/O) and (Hα/N) peak intensity ratios from the spectral scans correlated extremely well (R₂ ≻ 0.97) with local oxygen-based equivalence ratios. From the relations that were developed, it appears that for homogenous intake charge, such a LIBS diagnostic enables estimation of EGR rate with the knowledge of the global equivalence ratio.