Can we replace butane gas by methane gas?

no,propane can be replaced in place of methane.

No, Butane cannot be replaced by methane gas as butane gas is denser than methane gas.

Its depend,  as process or operation,  no we cannot use the same parameters with respect to operation .

As commercial use both transportation are totally different, but we can use as fuel vice versa . 

replace for what because each one has different properties especially for the BTU so please specify the question

we can so, there are some aspect that we have to keep in mind before to change it from the first one to the last one.... given that the chemical composition , calorific power and  composition liquid or gas vary , we should in some cases change tenchnical conditions, units, or facility in order to ensure the right use of the new fuel

Here are some point in my mind of changing the fuel gas for turbine. 

in gas turbine industry the manufacturing is design the internal hardware like liner based on fuel.

the fuel is not 100% of one gas in combination of gasses even if it small part. 

changing the gas might decree the life cycle of the hardware if it not within the design. it also will effect the emission of the machine. 

in DLN turbine changing the fuel might might make the tunning/mapping more difficult and might end with liner tuning for dilation hall.