Thermodynamic analysis of tri-reforming reactions to produce synthesis gas has been conducted by total Gibbs energy minimization to understand the effects of process variables, such as temperature (200-1000 degrees C), pressure (1-20 atm), and inlet O-2/CH4 (0-1.0), H2O/CH4 (0-3.0), and CO2/CH4 (0-3.0) mole ratios on the product distribution. The results reveal that high temperature and low pressure are favorable to achieve high H-2 production and CO2 conversion. In addition, excessive additions of H2O, OD and CO2 bring about lower H-2 yield and CO2 conversion, while low concentrations of H2O, O-2, and CO2 result in more intense carbon formation. To attain the maximum H-2 yield and high CO2 conversion coupled with a desired synthesis gas (H-2/CO) ratio for the downstream methanol production and effective elimination of carbon formation, the corresponding optimum feed ratio in tri-reforming process is identified to be CH4/CO2/H2O/O-2 = 1:0.291:0.576:0.088.