In　Integrated　Gasification　Combined　Cycle　(IGCC)　plant,　Gas　Turbine　(GT)　and　Air　Separation　Unit　(ASU)　have　great　influences　on　the　thermal　efficiency,　especially　their　interaction.　Therefore,　it　is　necessary　to　discuss　them　as　a　whole.　The　obstacle　to　achieve　this　is　the　complexity　of　the　double-column　ASU　modeling.　In　this　paper,　a　simultaneous　optimization　and　integration　methodology　is　developed　for　the　system　of　ASU　and　GT　to　increase　the　net　power　generation.　The　rigorous　mathematical　model　of　ASU　and　GT　is　proposed　as　the　basics.　Some　novel　simplifications　of　ASU,　such　as　the　limiting　temperature　difference　of　heat　transfer,　are　proposed　to　increase　the　model　flexibility　with　same　energy　prediction　accuracy.　After　the　validation　through　the　base　case,　two　improved　case　are　introduced　and　shows　additional　10%　and　14%　power　output　increment　compared　with　the　current　industrial　practice　(base　case).　(C)　2016　Elsevier　Ltd.　All　rights　reserved.