This two part-paper investigates the performance of SEWGS (Sorption Enhanced Water Gas Shift), an innovative system for CO₂ capture, applied to a coal fuelled Integrated Gasification Combined Cycle (IGCC). Firstly, two reference cases based on dry feed slagging Shell gasifier, with and without CO₂ capture, are defined. Conventional pulverized coal plant, where CO₂ capture is carried out with amine scrubbing technology, is also presented. Subsequently, SEWGS optimal integration in IGCC plant is investigated. The optimized parameters are SEWGS carbon capture ratio, CO₂ purity and number of vessels. Moreover, two different types of sorbent, Sorbent Alfa and Sorbent Beta, are considered in order to evaluate the impact of sorbent cyclic capacity on system performances. Both sorbents capture H₂S together with CO₂ with efficiency and equipment advantages. Results show that optimal SEWGS carbon capture ratio is in the range of 95% for both Sorbent Alfa and Sorbent Beta, with the latter achieving net electric efficiency of 40%. The resulting specific energy consumption for CO₂ avoided (SPECCA) is 2.0MJ/kg_CO2, which is 40% to 60% lower than reference IGCC and ASC reference cases respectively. Even Sorbent Alfa, which features a lower capacity than Sorbent Beta, achieves a SPECCA of about 2.5MJ/kg_CO2 which is 30% lower than reference case, illustrating SEWGS advantages. Part B will discuss the economic assessment of the investigated cases in order to determine the CO₂ avoided cost. This activity is carried out under the FP7 project CAESAR financed by the European Commission.