afremont:
I think it's safe to consider the 1.8W and 25C to be mutually exclusive unless liquid cooling is involved.
The datasheet specifies a junction to ambient thermal resistance of 55C/W. At 25C ambient and 1.8W that would mean a junction temperature of 125C, which is the maximum.
I would also add that most modern LEDs shine very brightly on just less than 5mA. They last forever that way too. 25mA just leads to short lives.
Who said 25ma?