A plume is a cloud of smoke, dust etc which rises up into the airplume of smoke/dust/gas/spray etc. There are times when a white coloured plume or discharge can be seen coming from the flue terminal on condensing boilers. Pluming is the effect seen from the flue on modern condensing boilers, it appears as a white "steam like" discharge from the flue. It’s a result of modern high efficiency boilers running at a cooler temperature than older, less efficient boilers did.
It is safe however, there are a range of flue additions to remove the plume from the area. If you wish to replace your boiler contact 01752 546160.