Is this propane or natural gas?
The discolorization is commonly from the incomplete combustion...meansin something is wrong in the burning process. Either the pressure is off on the gas supply to burners, the burners are dirty, orifices are dirty or the heat exchanger has a crack.
The heavy condensation is a from the combustion process.
I would strongly recommend having the pressure/firing rate checked, make sure inducer motor fins are clean and clear and make sure there are no vent restrictions.