Stricter environmental legislation on emissions, product qualities, and the increased availability of heavier and sourer crudes are the main driving forces for refineries to use gasification technologies for the "bottom of the barrel" disposal into production of hydrogen and clean energy. However, economic viability needs to be fully proven. This paper takes the challenge of integrating gasification to an overall refinery. To achieve this, a four stage optimization strategy is developed. In the screening and scoping stage, the energy integration opportunities are explored. In site level optimization, the overall integration among refinery and gasification is considered to maximize the margin. In process level optimization, appropriate integration among gasification, hydrogen, and utility systems is derived to minimize the investment. Finally, the simultaneous optimization of site and process levels is carried out to trade off between benefits and investment. By applying this methodology to a refinery case study, significant marginal improvement is achieved with minimal investment.