How to combine two or more downloaded files when wget fails halfway through

If wget fails halfway through and gives you many files like XXX.tgz, XXX.tgz.1, XXX.tgz.1.1 ..., you can combine them by :

cat XXX.tgz XXX.tgz.1 XXX.tgz.1.1 > XXX_new.tgz

or if you are in a directory where resides only the downloaded files:

cat * > XXX_new.tgz