Continuous Emission Monitoring Systems (CEMS) are widely used for industrial flue gas monitoring to support environmental compliance and operational control. However, many industrial facilities in developing countries still depend on imported monitoring systems with high procurement and maintenance costs. This study presents the design, calibration, and field evaluation of a sensor-based CEMS developed using locally integrated mechanical, electronic, and software components. The developed prototype was configured for continuous monitoring of CO₂, SO₂, CO, NOₓ, and O₂ concentrations in industrial flue gas applications. Laboratory calibration was conducted using certified reference gases under controlled conditions to evaluate measurement accuracy, repeatability, linearity, and short-term stability. Field evaluation was subsequently performed at a coal-fired power plant (PLTU Pacitan, Indonesia) during a 14-day deployment under actual operating conditions involving fluctuating temperature, humidity, and particulate-laden flue gas streams. Calibration results showed that CO₂, SO₂, and O₂ measurements achieved deviations below 3% relative to certified reference concentrations, while the CO channel exhibited higher deviation during low-concentration testing. Statistical evaluation demonstrated low variability (CV <2%) and strong linearity (R² >0.99) for the evaluated gas channels under controlled calibration conditions. During field deployment, the gas conditioning subsystem maintained stable sensor operation despite variable industrial conditions. Comparative observations with plant operational monitoring data indicated acceptable consistency within preliminary engineering-scale operational tolerance ranges. The results demonstrate the technical feasibility of integrating locally assembled components into a functional multi-gas monitoring platform for industrial emission applications. However, the present study should be interpreted as a preliminary engineering-scale validation. Additional long-term deployment, extended drift analysis, and direct comparison with certified commercial CEMS are still required before broader industrial equivalence and regulatory certification claims can be established.