It may be possible but I believe that sensor is more suited to detect an IR source such as an IR emitting "soccer ball". There are other sensors out there that may be better for this task, such as a IR temperature sensor (more sensitive) as they may be better at discerning your target source from background. And that is a perfect segue into...
will background signals, like sunlight, interfere with the IR-signals from the tealight?
Yes. Any Infrared radiation-emitting source will be detected. Although according to the documentation (http://www.hitechnic.com/cgi-bin/commerce.cgi?preadd=action&key=NSK1042
), the sensor can be set for modulated mode which filters out "unwanted" sources. But, this will probably include the tealight as well.
Search for "NXT infrared Temperature Sensor" with your preferred search provider. I know that Dexter Industries has one because I have on on order so that I can write a LeJOS driver for it.