It is normal. The truck's catalytic converter causes a chemical reaction that will convert harmful exhaust gases to harmless ones. When HC (hydrocarbons) and CO (carbon monoxide) gases are exposed to hot surfaces inside the converter, the HC and CO combine with oxygen to become carbon dioxide (CO2) and water (H2O).
In addition to this, condensation builds up inside the exhaust system and then is forced out the tail pipe. Again, normal.