Condensation occurs when the flue gasses from combustion are chilled below the dew point.....whether it is natural gas or propane.
The higher the efficiency of the device the more condensation produced---due do the cooler flue gas.
Both natural gas and propane are hydrocarbons. Unrefined natural gas include butane, ethane, and propane and methane which is the largest component of 'natural gas'. After refining, the methane is what you are actually getting delivered to your home.
So when you are using 'natural gas' you are using methane.
Propane has almost 2 1/2 times higher BTU content than 'natural gas'
I operated commercial boilers as a profession....fuel oil fired, natural gas fired, propane fired and raw unrefined 'natural gas'.
I know and understand the properties of combustion and flue gasses.
All flue gas is corrosive.