Lake Kivu contains ˜50 million tonnes (MT) dissolved biomethane. Efficient use is problematic from massive associated CO2: ˜600 MT. Conventional extraction scrubs CO2 with ˜50% overall CH4 loss, and returns ˜80% CO2 into the deep lake, preserving a catastrophe hazard threatening >2 M people. Methods and systems are disclosed coupling: (1) efficient CH4+CO2 degassing; (2) optional oxyfuel power generation and CO2 power cycle technologies; and (3) CO2 capture, processing, storage and use in a utilization hub. The invention optimally allows power production with >2× improved efficiency plus cryo-energy storage and large-scale greentech industrialization. CO2-utilizing products can include: Mg-cements / building materials, algal products / biofuels, urea, bioplastics and recycled materials, plus CO2 for greenhouse agriculture, CO2-EOR / CCS, off-grid cooling, fumigants, solvents, carbonation, packaging, ores-, biomass-, and agro-processing, cold pasteurization, frack and geothermal fluids, and inputs to produce methanol, DME, CO, syngas, formic acid, bicarbonate and other greentech chemicals, fuels, fertilizers and carbon products.