From one source .......... ,
The Higher Calorific Value of Bituminous Coal is 17,000 - 23,250 KJ/Kg
The Higher Calorific Value of Methane is 55,530 KJ/Kg
If I was determining whether a piece of bread near to a flame of these fuels will toast quicker based on the same rate of combustion of fuel gravimetrically , I would suppose methane as being a better bet .
You are also supposing that natural gas is methane but it can be a mixture of a lot of things . Here are a few Higher Heating Values for some different Natural Gas mixtures .
From another source .......... ,
Algeria: 42,000 kJ/m³ , Bangladesh: 36,000 kJ/m³ , Canada: 38,200 kJ/m³ , Indonesia: 40,600 kJ/m³ ,
Netherlands: 33,320 kJ/m³ , Norway: 39,877 kJ/m³ , Pakistan: 34,900 kJ/m³ , Russia: 38,231 kJ/m³ ,
Saudi Arabia: 38,000 kJ/m³ , United Kingdom: 39,710 kJ/m³ , United States: 38,416 kJ/m³ , Uzbekistan: 37,889 kJ/m³
These values are Volumetric values at standard temperature and pressure . Maybe some Algerian Natural gas might toast the bread quickest .