This study investigated the feasibility of the enhanced efficiency of energy yields in the catalytic gasification processes of paper-reject sludge. The experiments were conducted by controlling the temperatures that ranged from 600 °C to 900 °C with the addition of the mineral catalyst (calcium oxide, CaO) with a ratio that ranged between 10 percent and 20 percent by weight. According to the experimental results, increasing gasification temperatures will enhance the lower heating value (LHV) of syngas in the gasification of paper-reject sludge. The LHV of syngas increased from 10.6 MJ/Nm3 to 21.1 MJ/Nm3 with increased temperatures from 600 °C to 900 °C. The energy density of syngas was also increased from 1.13 to 4.56, as gasification temperature operated between 600 °C and 900 °C. This implied that the energy utilization of syngas, produced by gasification, could increase approximately four times the energy content of paper-reject sludge. Besides this, the energy yield efficiency in gasification will be enhanced by the tested mineral catalyst. In the case of gasification temperatures operating from 600 °C to 900 °C, the LHV of syngas increased significantly from 13.04 MJ/Nm3 to 21.52 MJ/Nm3 by the tested mineral catalyst. Meanwhile, the results of the energy density as well as the carbon conversion rate in catalytic gasification indicated that the results were 2.8-4.59 and above 95%, respectively. The experimental results of this research confirm that it is possible to improve the efficiency of tar reduction and energy yields using a tested mineral catalyst and can promote the potential for renewable energy in the application of paper-reject sludge.