To find a name in the census, go to the appropriate Index file first. Find the name of the person in the index and note what page number that person is found on the census. Then, click on the appropriate file.
Researchers should remember that from 1818 to 1847, Gallatin County included the present county as well as the territory now included in Saline County, and the northeastern third or so of Hardin County (thus Cave-in-Rock Township). The dividing line between Gallatin and Pope counties ran along the south border of what is now Saline County to its southeastern corner and then diagonally southeastwards to the Cave-in-Rock (the village, which is west of the cave sat in Pope County).

1818 - 1820 Illinois Census Returns
By Margaret Cross Norton [Free book on Google]

1820 Federal Census

Pages 45-52
Pages 53-57

1820 Gallatin Co Census Film

1830 Federal Census Index

Pages 250-263
Pages 264-277
Pages 278-292

1830 Gallatin Co Census Film

1840 Federal Census

Pages 2-6 Pages 7-13
Pages 14-19
Pages 20-25
Pages 26-31
Pages 32-38
Pages 39-43
Pages 44-49
Pages 50-56
Pages 57-62
Pages 63-65

1840 Gallatin Co Census Film

1850 Federal Census - Partial
New Heaven, Wabash, Cane Creek, Bear Creek,
North Fork, Shawneetown, Eagle, Equality)

Index A-C
Index D-J
Index K-Q
Index R-Z
Pages 347A-358B
Pages 359A-371A
Pages 371B-383B
Pages 384A-396A
Pages 396B-408B
Pages 409A-416B

1860 Gallatin Co Census Film

1870 Gallatin Co Census Film

1880 Gallatin Co Census Film

1900 Gallatin Co Census Film

1910 Gallatin Co Census Film

1920 Gallatin Co Census Film

1930 Gallatin Co Census Film

1940 Gallatin Co Census Film on NARA