Methane, one of the six major greenhouse gases, poses a serious environmental problem with the potential for global warming 20 times that of CO2. Although a large amount of methane is generated worldwide, its recovery and utilization are very low. One of the environmentally friendly ways to use methane is the biological gas-to-liquids (Bio-GTL) process, in which methane is biologically converted to useful products by microorganisms. Methanotrophs are strains that can convert methane to methanol at ambient conditions using methane monooxygenase (MMO). Here, we report an efficient phase-transfer catalysis system for methane-to-methanol conversion using methanotrophs. The methanotroph used in the work is Methylomicrobium alcaliphilum 20Z of which the methanol dehydrogenase (MDH) enzyme is removed to enhance methanol accumulation. The phase-transfer catalysis system does not require any separation processes and facilitates the mass transfer of methane gas, thereby increasing the methanol productivity and lowering the production cost. The methanol productivity is 0.717 g/L/h, which is superior to the results reported to date. In addition, the use of a cellulose-membrane reactor system enables multiple biocatalytic reactions without a significant decrease in productivity.