You will need to first determine the total energy output (Heat of combustion) required for that much of a temperature change in water
Heat required = - (mass of water) x (specific heat of water) x (change in temperature)
Use the molar heat of combustion of methane to determine how many moles of methane is required and use your mole conversion equation to change that to a mass value.
There should be a tonne of websites explaining this.