An integrated gasification combined cycle system. In one embodiment (FIG. 2) a system (200) includes an ion transport membrane air separation unit (210) for producing oxygen-enriched gas (209) and oxygen-depleted air (227), a gasification system (5) for generating syngas with the oxygen-enriched gas (209), a gas combustor (234) for reacting the syngas (224), and a subsystem configured to provide a first stream of air to the combustor (234) at a first pressure and to provide a second stream of air to the air separation unit (210) at a second pressure greater than the first pressure. The subsystem includes a compressor (230) having multi-pressure outlets (203, 204).