Can you remove the lens from a SLR camera while there is film in the camera, without ruining the film?

Yes, cos sometimes you need to use different lenses for different shots. So you can remove the lens without ruining the film.

you can change its lens without any problem unless you release the shutter. because when you press the button light goes inside the film and ruins it.

Yesx, but don't let anything touch the shutter mechanism exposed by removal of the lense.

Yes you can and I've been doing for over 30 years without the loss or damage to the neg/image. Just don't point the body towards any bright light just to be on the safe side.

Light is entering the camera body anyway at all times through the lens. The film is protected or covered by the mirror which replects the light up into the eyepiece. The mirror uncovers the film (or sensor in DSLR's) when you activate the shutter. So, as the others have answered, yes you can. That is the way these cameras were designed; to allow te lens to be changed at any time.

DSLR's are the same way.

SLR's are designed for lens interchangeability. After the picture is taken, the shutter curtain closes and the mirror drops down in front of it. The actual film is protected behind these two layers of design.
http://electronics.howstuffworks.com/cam...

Hope this is helpful.